An extrusion die is the part of an extrusion line or process responsible for shaping extrudable material (typically molten thermoplastic material) into a desired shape with the desired dimensions, usually as a sheet, film, curtain, pipe, or other elongate article.
Thermoplastic sheets are extruded by extrusion dies often referred to as “flat extrusion dies.” These dies include an inlet that receives an extrudable fluid material (typically molten thermoplastic material) that joins to a manifold expanding laterally to the width of the extruded sheet. The manifold joins a preland portion having a height that reduces gradually to the ultimate required thickness of the extrudate.
The manifold cavity and the preland heights and lengths are chosen in order to balance the pressure and velocity of the molten material as it exits the die. To determine the internal die dimensions of the manifold and preland portion, to obtain the required pressure and velocity balance, one needs to solve the flow equations, including the material equation, which relate the material viscosity and shear rate. A die that is balanced for a certain material may not be balanced for a different one.
A solution for a balanced die independent of the molten material was introduced by H. H. Winter and H. G. Fritz in 1984, and is referred to as the “Winter die.” The Winter design provides a constant shear rate throughout the die; that is, the shear rate is the same at each internal point in the Winter die. The Winter die design also provides a constant residence time throughout the die, so that the residence time for a particle flowing through the die is the same as that of a particle travelling straight through at the die centerline. Based on this, the Winter die design allows the extrusion of any material though the Winter die without affecting the die balance. The Winter die, however, could be improved.